It is useful for a number of physical and chemical processes to have available highly synchronized pulsed coherent radiation of differing optical frequencies. One is often faced with the requirement to excite a molecule or an atom with a combination of frequencies within a short time period in order to effect transitions to a given state of a molecular system, e.g., to achieve selectivity in an isotope separation process or to prepare a system for a chemical reaction. Although many processes can be excited stepwise and sequentially in time, it is usually the case that the most efficient stepwise excitation for radiatively connected states occurs with both radiation fields simultaneously applied. This is because in most cases collisional, radiative or transit-time decay reduces the population of the available species for the second step of excitation. The ability to synchronize two or more laser sources can be the most significant factor in determining the efficiency of such processes.
In general, it is difficult to obtain a high degree of synchronization between short pulses of laser light. For electrically excited gaseous discharge lasers which use thyratron control of the discharge firing, the discharges are initiated within a few nanoseconds. However, subsequent fluctuations in the buildup time of the optical pulses, which depend upon the resonant conditions, the electrical current, and the gas compositions and densities, produce typical overall interpulse jitter of from 10 to 50 nanoseconds. For generation of synchronized pulses shorter than 50 ns in duration, this degree of jitter would be unacceptable.
We now consider the generation of optical pulses through stimulated Raman scattering where the relative jitter between the output pulses is directly attributable to the jitter in the optical pump pulses which initiate the process. When a strong excitation pulse (Pump Pulse) is applied to a medium which is Raman responsive, stimulated scattering of the pump pulse results in the growth of radiation (Stokes pulse) which is downshifted by the Raman frequency of the medium and propagates in the pump direction. (In general, a Stokes pulse may grow in the opposite direction, but the physical structures that we shall consider do not allow such a pulse to develop.) The copropagating Stokes pulse is initiated by spontaneous Raman scattering and grows at an exponential rate that is proportional to the intensity of the pump pulse until pump depletion occurs or the medium has been traversed terminating the process. Energetically, for every quantum of energy appearing at the Stokes frequency a quantum of energy is removed from the pump and the energy difference defined by the Raman frequency is absorbed by the medium. This energy appears as an additional nonpropagating coherent excitation of the medium, known as the Raman wave, which provides a coupling mechanism between the two propagating electromagnetic waves. In general, within the limits imposed by dispersion, the Stokes pulse will copropagate with the depleted pump radiation and will become most intense at a point near the initial peak of the pump radiation. There is an intensity threshold for the appearance of the Stokes pulse. The threshold first occurs near the peak of the pump pulse. Usually, because of the limitations in gain or the presence of competing scattering processes, the width of the Stokes pulse is considerably less than that of the pump and the photon conversion efficiency is much less than 100%.
If a third electromagnetic wave at a frequency greater than the Raman frequency is injected colinearly with the pump wave when the Raman wave is present, a fourth electromagnetic wave shifted by the Raman frequency will be generated even if the intensity of the third wave is in itself insufficient to produce threshold gain for conversion to Stokes radiation. The fourth wave is produced by scattering from the Raman wave already present and therefore does not require the high gain needed to build from noise to threshhold. This physical scattering process is commonly called four wave mixing. Four wave mixing occurs only where the Raman wave overlaps the new pump (third) wave both in time and space within the limits of the transient response time of the medium. Therefore, the fourth wave (new Stokes wave) is completely overlapped (synchronized) with the Stokes wave generated in response to the pump wave. However, it can be seen that if the new pump (third) wave has a duration which is long compared with the first pump, the overall efficiency for the four wave mixing process will be small because only a small fraction of the total energy of the new pump will be converted. Additionally, in the presence of jitter, the two pump peaks may be displaced in time, further degrading the conversion efficiency.
For many useful Raman media, the stimulated Raman gain at practical pump laser intensities is insufficient for the Stokes radiation to reach threshold when starting from quantum noise in a single focused pass through the medium. It has been shown in U.S. patent application, Ser. No. 25,401, filed Mar. 30, 1979 (now U.S. Pat. No. 4,245,171) (this patent is incorporated herein by reference as though fully set forth herein) which issued to Messrs. Rabinowitz and Stein and which is entitled "Device for Producing High-Powered Radiation Employing Stimulated Raman Scattering in an Off-Axis Path Between a Pair of Spherical Mirrors" and also in an article entitled "Efficient tunable H.sub.2 Raman Laser" by Messrs. Rabinowitz, Stein, Brickman and Kaldor, which appeared in Applied Physics Letters, 35 (10), Nov. 15, 1979 at page 739 (which article is incorporated herein by reference as though fully set forth herein), that a multiple pass cell may be used to increase the cumulative gain so that threshold may be reached with pump intensities far below those required for a single pass device.
It was also found, as described in an article entitled "Controllable Pulse Compression in a Multiple-Pass-Cell Raman Laser", by Messrs. Perry, Brickman, Stein, Treacy and Rabinowitz, which appeared in Optics Letters, Volume 5, No. 7, July 1980 at page 288 (which article is incorporated herein by reference as though fully set forth herein), that in such a multiple pass cell, the energy in the Stokes pulse is compressed in time in comparison with the pump pulse as a result of ray crossings associated with the geometry of the multiple pass cell. In such a device, provided the length of the pump pulse extends over several passes of the cell, a forward propagating pump wave will intersect itself many times on each pass, and the intersection regions of the pump will be spaced at regular time intervals as indicated in Table I of the Perry et al article on page 289. During the initial buildup of the Stokes radiation from noise, the beam crossings have only small effect on the Stokes growth and threshold is reached near the peak of the pump pulse after a number of passes. The resulting Stokes pulse then interacts strongly at crossings with the intersecting pump radiation as allowed by the cell geometry. This interaction has two effects. First, energy is extracted from regions of the pump that are spaced away from the region where threshold is first reached by the time intervals between crossings. This suppresses the growth of copropagating Stokes radiation in those regions. Second, as the Stokes wave continues to grow, the threshold region broadens until its width is limited by the pump depletion produced by the crossings. Thus, a single Stokes pulse develops and has a width determined by the time difference between the peak and the nearest crossing (approximately 8/3 passes of the multiple pass cell). This pulse extracts energy from regions of the pump which would not have reached threshold in a device lacking crossings, but having the same cumulative gain. For strong interactions to occur at the crossings, it is necessary that the gain due to the intersecting pump be high. Such gain is maximized for a given intensity of the intersecting pump when the Raman gain is isotropic, and when the length of the crossing region is maximized.
In an article entitled "16-.mu.m Generation by CO.sub.2 -Pumped Rotational Raman Scattering in H.sub.2 " by Messrs. Byer and Trutna, appearing in Optics Letters, Volume 3, No. 4, October 1978 at page 144 (which article is incorporated herein by reference as though fully set forth herein), an apparatus was described in which four wave mixing occurs in a multiple pass cell. A Nd:Yag laser is employed to generate a Raman wave in parahydrogen to initiate Stokes radiation from a CO.sub.2 laser pulse which is simultaneously passed therethrough. The Nd:Yag laser pulse is short in time compared to the single pass transit time of the multiple pass cell.